A) Field of the Invention
The present invention relates to a high frequency filter having a resonance pattern of a microstrip line or a strip line structure.
B) Description of the Related Art
FIG. 13A is a plan view of a conventional high frequency filter, and FIG. 13B is a cross sectional view taken along one-dot chain line 13B-13B in FIG. 13A (JP-A-2006-115416).
On a principal surface of a dielectric substrate 101 (FIG. 13B), a resonance pattern 102, an input port 103 and an output port 104 (FIG. 13A) are formed. The resonance pattern 102 has a circular plan shape as illustrated in FIG. 13A. The input port 103 and output port 104 are electromagnetically coupled to the resonance pattern 102 at two points on a circumference of the resonance pattern 102 and on two radii intersecting with each other at a right-angle as illustrated in FIG. 13A. On the bottom surface of the dielectric substrate 101, a ground film 105 is formed as illustrated in FIG. 13B. The resonance pattern 102, ground film 105 and dielectric substrate 101 constitute a microstrip line.
Another dielectric substrate 110 is placed on the resonance pattern 102 as illustrated in FIG. 13B. On the surface of the dielectric substrate 110, a conductive pattern 111 is formed. The conductive pattern 111 is disposed at a position superposing upon a center point of an arc having a center angle of 270°, one end of the arc being a coupling position between the input port 103 and resonance pattern 102 and the other end of the arc being a coupling position between the output port 104 and resonance pattern 102. Plan shape of the conductive pattern 111 is, for example, a circular shape, and a diameter of the conductive pattern 111 is equal to or shorter than quarter of an effective wavelength of a high frequency signal propagating along the microstrip line.
Degeneration of two electromagnetic field modes of the resonance pattern 102 mutually crossing at a right angle is resolved and the resonance frequency is separated because the conductive pattern 111 and resonance pattern 102 are electromagnetically coupled with each other. In this state, the high frequency device shown in FIG. 13A functions as a dual mode filter.
As compared with a hair pin type resonance pattern and a straight line type resonance pattern, in the disc type resonance pattern shown in FIGS. 13A and 13B, current concentration upon a specific area is hard to occur. As compared also with a disc pattern having a notch at a disc circumference, current concentration upon a specific area is inhibited. Power tolerance of the disc type resonance pattern shown in FIGS. 13A and 13B is therefore high. The disc type resonance pattern shown in FIGS. 13A and 13B is expected to be applied to a transmission filter.
The characteristics of the high frequency filter shown in FIGS. 13A and 13B deviate from design values because of an air gap generated between the resonance pattern 102 and overlying dielectric substrate 110, a position displacement between the resonance pattern 102 and conductive pattern 111, and the like.